Corn-stover ethanol could increase CO2 emissions

Removing corn residues for production of cellulosic ethanol puts carbon into the air rather than back into the soil, and likely would generate more CO2 emissions than gasoline, at least initially, according to a new study from the University of Nebraska.

The federal government has been encouraging, and funding, development of technology for producing biofuels from high-fiber or “cellulosic” sources such as wood, grasses and crop residues such as corn stalks.

The researchers, led by Adam Liska, PhD, from the biological systems engineering department at the University of Nebraska, found that removal of corn residue for biofuels can decrease soil organic carbon and increase CO2 emissions because carbon in biofuels is oxidized to CO2 at a faster rate than when added to soil.

The team used a supercomputer model at UNL's Holland Computing Center to estimate the effect of residue removal on 128 million acres across 12 Corn Belt states. Their model indicates total annual production emissions from corn-residue biofuels, averaged over five years, would equal about 100 grams of carbon dioxide per megajoule – 7 percent greater than gasoline emissions and 62 grams above the 60 percent reduction in greenhouse gas emissions as required by the 2007 Energy Independence and Security Act.

The degree of impact on soil and air would depend on how much crop residue is harvested for biofuel production. "If less residue is removed, there is less decrease in soil carbon, but it results in a smaller biofuel energy yield," Liska says.

The study, in progress since 2007, was funded through a three-year, $500,000 grant from the U.S. Department of Energy. It used carbon dioxide measurements taken from 2001 to 2010 to validate a soil carbon model that was built using data from 36 field studies across North America, Europe, Africa and Asia.